Light detector having a photodiode with bias control

Abstract

The detector comprises an avalanche photodiode for receiving a light beam, a circuit for measuring the mean current across the avalanche photodiode and a bias voltage source. That source is controlled by the circuit for applying to the photodiode a bias voltage such that the mean current across the diode be substantially equal to an adjustable set value.

I claim: 
     
       1. A light detector for delivering an electrical measurement signal having signal components at a frequency representative of a frequency present in a light beam, having: an avalanche photodiode for receiving the light beam; source means for applying an adjustable bias voltage to said photodiode, whereby a variable measurement current is delivered by said photodiode; a circuit for measuring a mean value of said measurement current and delivering an output signal representative of said mean value; and means responsive to said output signal and controlling said source means for maintaining said mean value at a predetermined set value lower than that for which the current-voltage characteristic of said avalanche photodiode exhibits a bend. 
     
     
       2. Detector according to claim 1, wherein said converter further comprises voltage adjustment means connected to prevent the output voltage of said converter from lowering beyond a predetermined minimum value by overriding control by said transistor circuit. 
     
     
       3. A measurement assembly having: a light source delivering a light beam; means for separating said light beam into a first fraction and a second fraction and for directing said second fraction along a path where it is subjected to a physical phenomenon to be studied which modifies the frequency of light in said second fraction, said second fraction having an intensity much lower than that of the first fraction; an avalanche photodiode located to receive both said fractions; electrical source means for applying a bias voltage to said avalanche photodiode and causing a light representative current to pass in said photodiode; a circuit for measuring an average value of said current and delivering an electrical output signal representative of said average value; and means responsive to said output signal and arranged for controlling said source means for maintaining said average value at a predetermined set value lower than the current for which the inverse current-voltage characteristic of said photodiode exhibits a bend. 
     
     
       4. Detector according to claim 1, wherein said means responsive to the output signal of said circuit further comprises limiting means arranged for limiting said bias voltage to a predetermined maximum value. 
     
     
       5. Detector according to claim 1, wherein said circuit for measuring the mean value has a low pass filter having a cut-off frequency greater than 1 Hertz and in the 1 Hertz order of magnitude. 
     
     
       6. Detector according to claim 1, wherein said circuit for measuring the mean value comprises an integrating differential amplifier connected to receive an output signal from said photodiode and a reference voltage representing said set value and wherein said means responsive to the output signal of said circuit comprise a DC/DC converter and a transistor circuit having a control input connected to the output of said differential amplifier and an output connected to said converter for controlling said converter.